EIGRPCCNA 200-301

EIGRP Unequal-Cost Load Balancing Not Working

Presenting Symptom

EIGRP unequal-cost load balancing is not working; traffic is only using the best path even though feasible successors exist.

Network Context

A small enterprise campus with two routers (R1 and R2) connected via two serial links (T1 and 512kbps). Both routers run EIGRP AS 100 on Cisco IOS 15.x. The goal is to load balance traffic across both links using the variance command, but only the higher-bandwidth path is utilized.

Diagnostic Steps

Check EIGRP topology table for feasible successors

show ip eigrp topology

P 10.1.1.0/24, 1 successors, FD is 28160
         via 192.168.1.1 (28160/2560), Serial0/0
         via 192.168.2.1 (40512000/28160), Serial0/1

Look for multiple entries for the same prefix. If only one entry appears, there is no feasible successor. If multiple entries exist, check the FD and AD values to ensure the second path is a feasible successor (AD < FD).

Verify variance configuration

show running-config | section router eigrp

router eigrp 100
 network 10.0.0.0
 variance 2

Ensure variance is configured. The variance value must be at least equal to the ratio of the best metric to the feasible successor metric. If variance is missing or too low, unequal-cost load balancing will not occur.

Check EIGRP metrics for each path

show ip eigrp topology 10.1.1.0/24

P 10.1.1.0/24, 1 successors, FD is 28160
         via 192.168.1.1 (28160/2560), Serial0/0
         via 192.168.2.1 (40512000/28160), Serial0/1

Compare the FD of the best path (28160) with the metric of the feasible successor (40512000). The ratio is 40512000/28160 ≈ 1438. A variance of 2 would not include the second path because 2 < 1438. The variance must be at least 1438 to include the second path.

Verify EIGRP routing table for load balancing

show ip route eigrp

D    10.1.1.0/24 [90/28160] via 192.168.1.1, 00:00:05, Serial0/0
                  [90/40512000] via 192.168.2.1, 00:00:05, Serial0/1

If only one path appears, unequal-cost load balancing is not active. If both paths appear, the variance is correctly configured. In this case, only the best path appears because variance is too low.

Root Cause

The variance command is set to 2, but the feasible successor's metric (40512000) is 1438 times larger than the best path's FD (28160). Variance must be at least 1438 to include the second path. Unequal-cost load balancing fails because the variance value is too low to cover the metric difference.

Resolution

Increase the variance value to include the feasible successor. For this scenario, set variance to 1500 (or higher). Commands: ``` router eigrp 100 variance 1500 ``` This tells EIGRP to accept routes with a metric up to 1500 times the best path's FD. After applying, the second path will be installed in the routing table.

Verification

Run `show ip route eigrp` to confirm both paths are present: ``` D 10.1.1.0/24 [90/28160] via 192.168.1.1, 00:00:05, Serial0/0 [90/40512000] via 192.168.2.1, 00:00:05, Serial0/1 ``` Also run `show ip eigrp topology 10.1.1.0/24` to see both successors: ``` P 10.1.1.0/24, 2 successors, FD is 28160 via 192.168.1.1 (28160/2560), Serial0/0 via 192.168.2.1 (40512000/28160), Serial0/1 ```

Prevention

["When configuring variance, calculate the required value based on the metric ratio of the best path to the feasible successor.","Use the 'show ip eigrp topology' command to verify feasible successors and their metrics before setting variance.","Consider using traffic-shaping or adjusting EIGRP metrics (e.g., bandwidth, delay) to make unequal-cost load balancing more practical."]

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why unequal-cost load balancing is not working. The exam tests understanding of the variance command and the concept of feasible successors. Key fact: variance must be set to a value that includes the metric of the feasible successor; otherwise, only the best path is used.

Exam Tips

Remember that variance only works if the alternate path is a feasible successor (AD < FD).

The variance value is a multiplier; a variance of 1 means equal-cost load balancing only.

Know the 'show ip eigrp topology' command to verify feasible successors and their metrics.

Commands Used in This Scenario

show ip eigrp topology

Displays the EIGRP topology table, showing all learned routes and their feasible successors, used to verify EIGRP convergence and path selection.

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